fix: download vantage-shared before asking the module cache where it is

go list -m -f '{{.Dir}}' reports an empty Dir and exits 0 for a module that
is not in the cache, so on a cold runner swag was handed an empty --dir and
failed several steps later with 'dir:  does not exist'. Download first, and
fail loudly if the path is still not there.

.gitea/workflows/server-deploy.yml

@@ -134,7 +134,17 @@ jobs:
                   # shared module's files on disk. They are in the module cache
                   # now rather than at ../shared, and the cache is read-only,
                   # which swag does not mind.
+                  #
+                  # go mod download first: `go list -m -f {{.Dir}}` reports an
+                  # empty Dir and exits 0 for a module that is not yet in the
+                  # cache, so on a cold runner swag was handed an empty path and
+                  # failed with "dir: does not exist" several steps later.
+                  go mod download gitea.hostxtra.co.uk/vantage/vantage-shared
                   SHARED_DIR="$(go list -m -f '{{.Dir}}' gitea.hostxtra.co.uk/vantage/vantage-shared)"
+                  if [ -z "$SHARED_DIR" ] || [ ! -d "$SHARED_DIR" ]; then
+                      echo "vantage-shared source not in the module cache: '$SHARED_DIR'" >&2
+                      exit 1
+                  fi
                   swag init --generalInfo cmd/main.go --dir "./,$SHARED_DIR" \
                     --output internal/api/docs --outputTypes json --v3.1
                   mv -f internal/api/docs/swagger.json internal/api/docs/openapi.json